FIG. 1 depicts a portion of a prior art reflective (i.e. front-lit) frustrated total internal reflection (TIR) modulated display 10 of the type described in U.S. Pat. Nos. 6,885,496; 6,891,658; 7,760,417 and 8,040,591. These patents describe an entirely new design of the outward sheet that was previously described in U.S. Pat. Nos. 5,959,777; 5,999,307; 6,064,784; 6,215,920; 6,304,365; 6,384,979; 6,437,921; 6,452,734 and 6,574,025 which comprised of, for example, various spatially uniform prism structures, dielectric light fibers, parallel, and perpendicular and interleaved structures. As a result of the closely packed, spherical or hemi-spherical beaded, outward sheet design first described in patents ‘496’ and ‘658’, the practical angular viewing range of frustrated TIR or other reflective display methods was increased. The new design offers semi-retro-reflective gain, whereby light rays which are incident on the surface of the convex protrusions in the shape of hemispherical beads are reflected back (but not exactly retro-reflected) toward the light source; which means that the reflection is enhanced when the light source is overhead and slightly behind the viewer, and that the reflected light has a diffuse characteristic giving it a white appearance, which is desirable in reflective display applications.
Display 10 in FIG. 1 includes a transparent outward sheet 12 formed by partially embedding a large plurality of high refractive index (e.g. η1>˜1.90) transparent spherical or approximately spherical beads (it is noted that said spherical or approximately spherical beads may also be “hemi-spherical beads” or “hemi-beads” or “beads” or “hemi-spherical protrusions” or “hemi-spheres” or “convex protrusions” and these terms may be used interchangeably) 14 in the inward surface of a high refractive index (e.g. η2≈η1) polymeric material 16 having a flat outward viewing surface 17 which viewer V observes through an angular range of viewing directions Y. The “inward” and “outward” directions are indicated by double-headed arrow Z. Beads 14 are packed closely together to form an inwardly projecting monolayer 18 having a thickness approximately equal to the diameter of one of beads 14. Ideally, each one of beads 14 touches all of the beads immediately adjacent to that one bead. Minimal interstitial gaps (ideally, no gaps) remain between adjacent beads.
An electro-active TIR-frustrating medium 20 is maintained adjacent the portions of beads 14 which protrude inwardly from material 16 by containment of medium 20 within a reservoir 22 defined by lower sheet 24. An inert, low refractive index (i.e. less than about 1.35), low viscosity, electrically insulating liquid such as, but not limited to, Fluorinert™ perfluorinated hydrocarbon liquid (η3˜1.27) available from 3M, St. Paul, Minn. is a suitable fluid for the medium 20. Other liquids such as Novec™ also available from 3M may also be used as the fluid for medium 20. A bead:liquid TIR interface is thus formed. Medium 20 contains a finely dispersed suspension of light scattering and/or absorptive particles 26 such as inorganic or organic pigments, dyes, dyed or otherwise scattering/absorptive silica or latex particles, etc. Sheet 24's optical characteristics are relatively unimportant as sheet 24 need only form a reservoir for containment of electro-active TIR-frustrating medium 20 and particles 26, and serve as a support for backplane electrode 48.
In the absence of TIR-frustrating activity, as is illustrated to the right of dashed line 28 in FIG. 1, a substantial fraction of the light rays passing through sheet 12 and beads 14 undergoes TIR at the inward side of beads 14. For example, representative incident light rays 30 and 32 are refracted through material 16 and beads 14. The light rays undergo TIR two or more times at the bead:liquid TIR interface, as indicated at points 34 and 36 in the case of ray 30; and indicated at points 38 and 40 in the case of ray 32. The totally internally reflected rays are then reflected back through beads 14 and material 16 and emerge as rays 42 and 44 respectively, achieving a “white” appearance in each reflection region or pixel.
A voltage can be applied across medium 20 via electrodes 46 and 48 which can for example be applied by, for example, vapor-deposition to the inwardly protruding surface portion of beads 14 and to the outward surface of sheet 24. Electrode 46 is transparent and substantially thin to minimize its interference with light rays at the bead:liquid TIR interface. Backplane electrode 48 need not be transparent. If TIR-frustrating medium 20 is activated by actuating voltage source 50 to apply a voltage between electrodes 46 and 48 as illustrated to the left of dashed line 28, suspended particles 26 are electrophoretically moved into the region where the evanescent wave is relatively intense (i.e. within about 0.25 micron of the inward surfaces of inwardly protruding beads 14, or closer). When electrophoretically moved as aforesaid, particles 26 scatter or absorb light, thus frustrating or modulating TIR by modifying the imaginary and possibly the real component of the effective refractive index at the bead:liquid TIR interface. This is illustrated by light rays 52 and 54 which are scattered and/or absorbed as they strike particles 26 inside the thin evanescent wave region at the bead:liquid TIR interface, as indicated at points 56 and 58 respectively, thus achieving a “dark” appearance in each TIR-frustrated non-reflective absorption region or pixel. Particles 26 need only be moved outside the thin evanescent wave region, by suitably actuating voltage source 50, in order to restore the TIR capability of the bead:liquid TIR interface and convert each “dark” non-reflective absorption region or pixel to a “white” reflection region or pixel.
As described above, the net optical characteristics of outward sheet 12 can be controlled by controlling the voltage applied across medium 20 via electrodes 46 and 48. The electrodes can be segmented to electrophoretically control the particles suspended in the TIR frustrating, low refractive index medium 20 across separate regions or pixels of sheet 12, thus forming an image.
As shown in FIGS. 2A-2G of a closer examination of an individual hemi-bead 60, reflectance of a the hemi-bead 60 is maintained over a broad range of incidence angles, thus enhancing display 10's wide angular viewing characteristic and its apparent brightness. For example, FIG. 2A shows hemi-bead 60 as seen from perpendicular incidence—that is, from an incidence angle offset 0° from the perpendicular and r is the radius of the hemispherical bead. In this case, the portion 80 of hemi-bead 60 appears as an annulus. The annulus is depicted as white, corresponding to the fact that this is the region of hemi-bead 60 which reflects incident light rays by TIR, as aforesaid. The annulus surrounds a circular region 82 which is depicted as dark, also referred to as the “pupil”, corresponding to the fact that this is the non-reflective region of hemi-bead 60 within which incident rays pass through and may be absorbed and do not undergo TIR. FIGS. 2B-2G show hemi-bead 60 as seen from incident angles which are respectively offset 15°, 30°, 45°, 60°, 75°, and 90° from the perpendicular. Comparison of FIGS. 2B-2G with FIG. 2A reveals that the observed area of reflective portion 80 of hemi-bead 60 decreases only gradually as the incidence angle increases. Even at near glancing incidence angles (e.g. FIG. 2F) an observer will still see a substantial part of reflective portion 80, thus giving display 10 shown in FIG. 1 a wide angular viewing range over which high apparent brightness is maintained.